A lower frequency band of less than 6 GHz, which is mainly used in a current mobile communication, is fully used or fragmented with existing systems such as mobile communications, broadcasting, and satellite communications, which makes it difficult to secure ultra-wide bandwidth frequency for high-capacity data transmission.
As a result, a higher frequency band such as a centimeter wave of 6 GHz or above (an electromagnetic wave with a wavelength in centimeters corresponding to a frequency range of 3 to 30 GHz) or a millimeter wave (an electromagnetic wave with a wavelength in millimeters corresponding to a frequency range of 30 to 300 GHz) becomes a key spectrum of 5th generation.
Such a higher frequency band is advantageous for an implementation of an RF system, which is capable of widening operation bandwidth as a center frequency becomes higher, and it is possible to achieve a high density antenna. In other words, assuming a physical size is the same for each antenna in the higher frequency band, a physical distance between the radiators constituting the antenna is reduced as the frequency is higher. Therefore, a large number of radiators can be integrated in the antenna. Such radiators becomes a hardware basis for 3D beamforming that controls an amplitude and a phase of an RF signal to generate various types of antenna beams and massive multiple input multiple output (MIMO) technology that enables multiple transmissions.
Meanwhile, while the higher frequency band is advantageous in that a wide frequency band can be used therein compared to a cellular band as discussed above, there is a drawback to overcome high linearity and low diffraction characteristics of the higher frequency band and a relatively higher path loss. To this end, the increased path loss is overcome by forming a pencil beam having a high gain by utilizing a plurality of antenna radiators increased due to the use of a high frequency.
However, in the higher frequency band, a beam width becomes very narrower and the linearity of the electromagnetic waves becomes higher while the diffraction thereof becomes lower. Therefore, smooth communications cannot be achieved when the base station and the terminal are not able to utilize appropriate transmission/reception beams according to a location change of the terminal.
FIG. 1 shows an example of an antenna operation scheme in a conventional lower frequency band-based wireless communication system. FIG. 2 shows an example of an antenna beam pattern in the conventional lower frequency band-based wireless communication system.
Referring to FIGS. 1 and 2, the conventional lower frequency band-based wireless communication mainly utilizes a lower frequency band of less than 6 GHz in which path attenuation, i.e., path loss, of electromagnetic waves depending on a transmission/reception distance between a base station and a terminal is reduced. Thus, even when an antenna having a relatively wide beam width is applied thereto, a smooth communication link can be generated.
However, when an antenna having a wide beam width is used in a higher frequency band of 6 GHz or above which may be suitable for high-capacity data transmission in 5th generation of mobile communication, received electric field cannot be ensured due to high path loss.
FIG. 3 illustrates a conventional scheme for deriving an optimum antenna beam in the higher frequency band-based wireless communication. The scheme shown in FIG. 3 is a beam switching scheme for selecting a beam that guarantees the best wireless link among a plurality of preset antenna beam group (e.g., a base station antenna reception beam #1 to a base station antenna reception beam #N).
However, when the number of candidate antenna beams is small in a predefined antenna beam group or when the beam width of a candidate antenna beam is wide, there may be a problem that loss of electric field occurs even when electromagnetic waves are transmitted and received to a main lobe in the antenna beam.
Hereinafter, the problem in the conventional beam switching scheme will be described in more detail with reference to FIGS. 4 to 6.
FIG. 4 shows an example of an antenna beam pattern for each antenna beam index in the higher frequency band-based wireless communication. FIG. 5 is an enlarged view of a region between −10° and 10° shown in FIG. 4. The antenna beam pattern data shown in FIGS. 4 and 5 does not have continuous values, but discrete values at predetermined angular intervals.
FIG. 6 is a more specific illustration of the conventional scheme for deriving an optimum antenna beam in the higher frequency band wireless communication. In the conventional scheme for deriving the optimum antenna beam, as shown in FIG. 6, an amplitude of a reception signal received from the terminal for each antenna reception beam is compared with the others, and the antenna beam including a reception signal having the largest amplitude is selected as the optimum antenna reception beam. According to this scheme, when an angle of arrival (AoA) (i.e., an angle of a direction from the terminal to the base station with respect to the reference direction) of a signal from the terminal does not coincide with a directional angle (i.e., an angle at which the amplitude of the reception signal is maximum) of the selected optimum antenna reception beam (e.g., in FIG. 6, a base station antenna reception beam #3 is selected as the optimum reception beam, and the terminal is not located at a directional angle 0° of the base station antenna reception beam #3), the loss of the electromagnetic waves occurs as compared with the case where an angle of arrival of a signal from the terminal coincides with a directional angle of an optimum antenna reception beam. This loss can be generated by a difference between an amplitude of the electromagnetic wave at the directional angle of the optimum antenna reception beam and an amplitude of the electromagnetic wave at the intersection of the optimum antenna reception beam and the adjacent beam (base station antenna receive beam #2).
Conventionally, there is proposed another method to solve the problem of the beam switching scheme. Korean Patent Application Publication No. 2014-0065630 (published on May 30, 2014) discloses a method of obtaining a suitable beam by a recursive repetition in which an optimal beam between a base station and a terminal is first obtained by using an antenna beam having a wide beam width, and then the selected beam region is subdivided into antenna beams having narrow beam widths or a direction-of-arrival (DOA) method based on Maximum Likelihood (ML) to estimate a location of the terminal at each stage.
However, in the above described method, it is necessary to utilize the data received from the entire beam used in each stage and estimate the angle of the terminal (that is, the angle of arrival of the signal from the terminal) by a matrix calculation of the data, which requires a relatively large amount of calculation.